In common production processes for semiconductor devices, fine patterns have been formed by photolithography. The formation of a fine pattern usually requires many substrates called photomasks. A common photomask is composed of a translucent glass substrate having thereon a shielding fine pattern composed of a metal thin film. The production of the photomask involves use of a photomask blank composed of a translucent substrate such as a glass substrate having thereon a shielding film. The production of the photomask with the photomask blank includes an exposure step (or drawing step) of subjecting a resist film formed on a photomask blank to intended pattern exposure (or pattern drawing), a development step of developing the resist film to form a resist pattern according to the intended pattern exposure (or pattern drawing), an etching step of etching the shielding film along the resist pattern, and a removal step of removing the residual portions of the resist pattern. In the development step, the resist film formed on the photomask blank is subjected to intended pattern exposure (or pattern drawing), then a developing solution is fed to the resist film thereby dissolving the portions soluble in the developing solution, and thus a resist pattern is formed. In the etching step, the resist pattern is used as a mask, and the exposed areas on the shielding film having no resist pattern are dissolved by, for example, wet etching, and thus an intended mask pattern is formed on the translucent substrate. In this way, a photomask is produced.
Patent Document 1 describes a mask blank suitable for wet etching, wherein the mask blank is a photomask blank including a transparent substrate having thereon a shielding film composed of a chromium film containing chromium carbide. Patent Document 2 also describes a mask blank suitable for wet etching, wherein the mask blank is a halftone phase shift mask blank including a transparent substrate having thereon a laminated film composed of a halftone material film and a metal film. From the surface side toward the transparent substrate side of the metal film, there are regions composed of different materials etched at different rates. The halftone phase shift mask blank is composed of, for example, a CrN/CrC metal film (a metal film wherein CrN and CrC are laminated in this order from the transparent substrate side, hereinafter the same) and a CrON antireflection film.
Miniaturization of the pattern of a semiconductor device requires miniaturization of the mask pattern formed on the photomask, and shortening of the exposure wavelength used for photolithography. In recent years, exposure sources having shorter wavelengths have been used for the production of semiconductor devices, so that a KrF excimer laser (wavelength: 248 nm) has been replaced with an ArF excimer laser (wavelength: 193 nm), and then a F2 excimer laser (wavelength: 157 nm). On the other hand, regarding photomasks and photomask blanks, miniaturization of a mask pattern formed on a photomask requires the reduction of the thickness of the resist film on a photomask blank, and replacement of the conventional wet etching process with a dry etching process as the patterning technique in the photomask production.
However, the reduction of the thickness of the resist film and the dry etching process present the following technical problems.
The first is that the reduction of the thickness of a resist film on a photomask blank is significantly limited by the processing time of the shielding film. A shielding film is commonly made of a chromium-based material, and chromium is dry-etched using a mixed gas composed of chlorine and oxygen. When a shielding film is patterned by dry etching with a resist pattern as a mask, the resist is very susceptible to oxygen plasma contained in the dry etching atmosphere because the resist is an organic film composed mainly of carbon. During patterning of a shielding film by dry etching, the resist pattern formed on the shielding film must have an adequate thickness. As an index, in order to form a mask pattern having a good sectional shape, the resist film must have a sufficient thickness so as to remain after etching for a period of about twice the clear etching time (100% over-etching). For example, etching selectivity between chromium composing the shielding film and the resist film is usually 1 or less, so that the thickness of the resist film must be twice or more the thickness of the shielding film. The processing time of the shielding film may be reduced, for example, by reducing the thickness of the shielding film. The reduction of the thickness of the shielding film is proposed in Patent Document 3.
Patent Document 3 discloses a method for producing a photomask, wherein the shape of a chromium pattern is improved by reducing the thickness of a chromium shielding film on a transparent substrate thereby reducing the etching time.    Patent Document 1: Japanese Examined Patent Application Publication (JP-B) No. 62-32782    Patent Document 2: Japanese Patent (JP-B) No. 2983020    Patent Document 3: Japanese Unexamined Patent Application Publication (JP-A) No. 10-69055